This invention relates generally to game balls, and more particularly to a game ball, such as a golf ball, having an ionomeric cover.
Before the development of ionomers, balata was the preferred material for golf ball covers. Polyethylene also was proposed for use as a golf ball cover material but was generally deemed highly inferior to balata in imparting playability and durability characteristics to the ball due to its brittleness and high hardness, and thus never became a commercially successful golf ball cover material.
Balata golf ball covers have now been replaced to a great extent by ionomeric cover materials. Ionomers are copolymers of an olefin and an xcex1,xcex2-ethylenically unsaturated carboxylic acid with a portion of the carboxylic acid groups neutralized by a metal ion. The metal ions serve as crosslinking agents, as they are ionically bonded to carboxylic acid groups in adjacent copolymer chains. Instead of having thermally irreversible covalent bonding, ionomers have thermolabile crosslinking in which metal ions become part of the chemical structure of the ionomer upon crosslinking, and these crosslinks are reversible. For purposes of this application, this type of crosslinking is referred to as ionic crosslinking. One of the advantages of ionic crosslinking in golf ball materials is the ability of ionic bonds to re-form after breaking as a result of processing at elevated temperatures.
There are numerous advantages to the use of ionomers in making golf ball covers. On the other hand, one drawback of conventional golf balls with soft ionomeric covers are that the covers are prone to scuffing and cutting, particularly when hit with irons which have sharp grooves. It would be useful to develop a golf ball with a soft ionomeric cover which is highly resistant to cutting and scuffing by sharp-grooved clubs.
An object of the invention is to provide an ionomeric game ball cover having improved scuff resistance and/or cut resistance.
Another object of the invention is to provide a method for imparting improved scuff resistance and/or cut resistance to a game ball cover.
Yet another object of the invention is to provide a golf ball with a soft cover which is well-suited for use with golf club irons having sharp grooves.
Yet another object of the invention is to provide a method of forming a golf ball with a soft cover which has excellent scuff resistance and/or cut resistance.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
The invention in a preferred form is a game ball having a cover comprising an ionomer resin. The ionomer resin includes a copolymer of an olefin and an xcex1,xcex2-ethylenically unsaturated carboxylic acid which is about 10-100% neutralized with metal ions. The copolymer has a sufficient degree of covalent crosslinking to impart to the cover improved resistance to at least one of scuffing and cutting. In a particularly preferred form of the invention, the game ball is a golf ball. Preferably, the game ball has a dimpled surface. The covalent crosslinking preferably comprises irradiation-induced covalent crosslinking.
In a preferred form of the invention, the degree of covalent crosslinking is appropriate to impart to the cover a Shore D hardness which is no more than about 10% greater, and more preferably no more than about 5% greater, than the Shore D hardness of a cover having an identical composition but which does not include a substantial degree of covalent crosslinking. Preferably, the copolymer includes an acrylate.
Another preferred form of the invention is a method of treating a game ball. The method comprises the steps of obtaining a game ball having a cover comprising an ionomer, and irradiating the ionomer in the cover under conditions appropriate to covalently crosslink the ionomer in order to increase the resistance of the cover to at least one of scuffing and cutting without substantially impairing other playability characteristics of the ball. In a particularly preferred form of the invention, the game ball is a golf ball. Preferably, the game ball has a dimpled surface.
According to the preferred method of the invention, the game ball is subjected to electron beam treatment at a dosage of at least 2 megarads. The game ball cover preferably is irradiated prior to application of a top coat over a cover. The method of the invention preferably further includes the step of applying a top coat over the cover before or after irradiation.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others and the article possessing the features, properties, and the relation of elements exemplified in the following detailed disclosure.